29 replies to this topic

[Rob Himself]

I've got one! Got a mount on the gutter rail, nice 'n' solid. ... pics to follow tonight.
Only problem is that it's mounted by passenger's knees, so I can only stretch + reach to adjsut volume/scan the channels.

Listening in + chatting to truckers is funny lol

Edited by Rob Himself, 17 December 2007 - 02:47 PM.

[Silicon Skum]

Didn't get time to take pics, will do it in the morning (too dark to see the antenna now).


Gutter mounts can be a problem for range, as I said, the signal is pulled towards the area of the car that has the most metal in front of the antenna. So instead of the signal range being equel in all directions (like a ball expanding out from the car) with the antenna mounted central on the roof, the shape of the signal will change to a more directional shape (sort of cone shaped, and aimed towards the oposite side of the car to the antenna mount). So side mounted gutter mounts are bad as you will only transmit and receive MOST of the signal in the direction the other side of your car is facing. In most cases the gutter mount should be mounted at the rear of the car, so most of the signal is aimed forward of the car (in the direction you are driving - which is better for convoy use........unless your at the front, then your signal will be poorer).


Side panel / boot mounts, or bumper mounts are BAD for range, even if it means you can use a longer antenna, as a good section of the antenna is sheilded by the car body. Can also give very erratic signal quality.

SS

[redhotmini]

i use a 4 foot 'orbiter' springy ariel for close convoy stuff, and a 7 foot twin coil job (cant remember the name) for long distance, with a burner as well if i need to. i just have a huge mag mount, although the big ariel needs some support, so ive mounted that to a old school roof rack. should be useful on the lands end to john o groats run next year.

[stardude]

anyone managed to get any pics yet?

[paulrockliffe]

It won't matter where the aerial is on a fibreglass car though will it? Should I get ann artificial ground plane thingy, or just earth it to the battery?

Ta.

[Silicon Skum]

anyone managed to get any pics yet?

OOPS! completly forgot about this.

Yeah, I managed to get some pics using the Bini - will post 'em up in the morning. Can prolly get a few with the proper mini in a few days (after crimbo) which will show the height of the antennas better, but the Bini roof isn't too far off the size of the mini roof.

It won't matter where the aerial is on a fibreglass car though will it? Should I get ann artificial ground plane thingy, or just earth it to the battery?

Yeah you need to have a METAL ground plane, it forms one half of the antenna circuit ( it works like a reflection of the antenna, which allows the antenna to effectivly be larger than it's pysical size. Your SWR would be miles off, and you risk blowing the final stage RF transistor in the CB.

There are antennas that don't require a ground plane, but they are not as effective. There are other options though, 4 lengths of wire can be used to form a ground plane, or use a large metal (steel or copper) sheet under the GF panel as the ground plane (will also work using a mag mount and steel sheet, so no need to drill the panel for an electrcal connection - mag mounts capacitively couple to the metal sheet, and the magnet will hold the antenna in place.). You could also install co-phased (dual) antennas, if you can do so in a way that allows at least two-thirds of both antennas to be within clear, unobstructed view of one another. The co-phase options works very well because the transmission pattern is strongest towards the front and rear of the vehicle - so good for convoy use. You will need an SWR meter to set these up correctly.

Artificial ground plane antennas are prolly the easiest way for most people, there is a more info, and a few antennas here:

http://www.4x4cb.com...e.cfm?CatID=646

I would give the guy who runs the shops a call and explain what you have and what you need, he's very helpfull and knows his stuff.

And if you don't have one, pick up an SWR meter - you DO need one, even if the antenna is "pre-SWRed" or "pre-tuned", they STILL have to be set up with an SWR meter or the antenna will not radiate efficiently (and could overload the RF transistor of the CB).

SS

[paulrockliffe]

Ok, thanks, I mostly follow what you're saying, I'm not sure I like the idea of glueing a big sheet of steel above my head tho, it's bound to fall off lol!

Let me bounce some ideas off you:

One or two aerials grounded attached by metal brackets to the rear subframe? Would that work at all, the rear subframe is kinda like 4 pieces of wire. Please say yes, the aerials definitely look best if they're mounted at the rear corners as low as possible!

How thick a copper sheet would be needed? You can get sheet copper fairly thin then just gently hammer it tot he cruve of the roof and glue? The magnetic mount wouldn't stick to copper tho would it.

I have an SWR meter already, so that's not a problem.

Cheers.

[Yeah you need to have a METAL ground plane, it forms one half of the antenna circuit ( it works like a reflection of the antenna, which allows the antenna to effectivly be larger than it's pysical size. Your SWR would be miles off, and you risk blowing the final stage RF transistor in the CB.

There are antennas that don't require a ground plane, but they are not as effective. There are other options though, 4 lengths of wire can be used to form a ground plane, or use a large metal (steel or copper) sheet under the GF panel as the ground plane (will also work using a mag mount and steel sheet, so no need to drill the panel for an electrcal connection - mag mounts capacitively couple to the metal sheet, and the magnet will hold the antenna in place.). You could also install co-phased (dual) antennas, if you can do so in a way that allows at least two-thirds of both antennas to be within clear, unobstructed view of one another. The co-phase options works very well because the transmission pattern is strongest towards the front and rear of the vehicle - so good for convoy use. You will need an SWR meter to set these up correctly.

Artificial ground plane antennas are prolly the easiest way for most people, there is a more info, and a few antennas here:

http://www.4x4cb.com...e.cfm?CatID=646

I would give the guy who runs the shops a call and explain what you have and what you need, he's very helpfull and knows his stuff.

And if you don't have one, pick up an SWR meter - you DO need one, even if the antenna is "pre-SWRed" or "pre-tuned", they STILL have to be set up with an SWR meter or the antenna will not radiate efficiently (and could overload the RF transistor of the CB).

SS

[Silicon Skum]

Damn it! photos no good, two of my antennas are black (and so is my roof...) and didnt show up well. the other is grey and didnt show up well against the grey rendered wall........I think I need to adjust the camera settings.

I might just take a couple of pics of the antennas tonight and photoshop the antennas on to the mini.

Ok, thanks, I mostly follow what you're saying, I'm not sure I like the idea of glueing a big sheet of steel above my head tho, it's bound to fall off lol!

Let me bounce some ideas off you:

One or two aerials grounded attached by metal brackets to the rear subframe? Would that work at all, the rear subframe is kinda like 4 pieces of wire. Please say yes, the aerials definitely look best if they're mounted at the rear corners as low as possible!

How thick a copper sheet would be needed? You can get sheet copper fairly thin then just gently hammer it tot he cruve of the roof and glue? The magnetic mount wouldn't stick to copper tho would it.

I have an SWR meter already, so that's not a problem.

Cheers.

Ok, first off, you don'nt need a thick steel panel, just something similar to a normal mini panel (quite thin stuff) mounted under the GF panel (if using the roof panel, you could just cut a section of mini roof panel from a scrap shell and mount that). Copper sheet is available in the same thickness, and usually used for roofing purposes. And no, the magnet will not stck to the copper sheet, you would have to drill and mount the antenna connector to the GF panel and electrically connect the copper panel to the co-ax cable braid.

If you are using the TWO antenna method (co-phased), you don't need a seperate earth - One antenna is used as the ground.

Connecting the the subframe would work to some degree, but not really ideal. If the whole car body is GF, then I would say the better option would be to use a artificial ground plane such as this one:

http://www.4x4cb.com....cfm?itemID=972

They slightly limit the size and weight of the antenna you can use (because they are not THAT strong to be able to take a 7ft whip.....) but you will get good results with the correct choice of antenna. I would say a 3ft mini springer is the minimum, a 4ft DV27 would work well (very lightweight, could possibly run a 5ft if you can find one), 4ft Fire-Stick (max, they are quite heavy but VERY good), Sirio Mythos 9000 is also a very good antenna (there are a number of similar clone models also). The guy at at 4x4cb.com will be able to give better advice on the antenna - he knows his products better than I do.

There are also the "marine" (boats) type antennas, typically they are ALWAYS white, but there are a few other colours in some makes. These antennas work by using the co-ax cable braid as the the ground plane, so this means that tey come with a fairly long wire, and you CAN'T coil it up or cut it shorter, it needs to be laid straight and any excess looped around the interior of the car. The instructions will give you details on how to run the cable. These might be slightly cheaper, and will give similar results to the electronic ground plane.

Using these "no ground" antennas you would be best off using them aas high as possible - on the roof, I would say the best idea would be to use a mag mount, and another strong magnet (large speaker etc.) on the underside of the roof panel - the two magnets will pull together, and be VERY hard to shift. Works well for many people who have non metal roofed Landies.


SS

[paulrockliffe]

Thanks SS,

Will study your reply in detail post-Top Gear. In the mean time: How do you wire the two aerials together? I only have one aerial connection on my radio.

Cheers

[Silicon Skum]

Thanks SS,

Will study your reply in detail post-Top Gear. In the mean time: How do you wire the two aerials together? I only have one aerial connection on my radio.

Cheers

Ok, I see your keen on the twin antenna setup. This is not the most simple option, and I will have to do a couple of tests to see what works first. It's not something I've actually used before (never had a need). Normally twin antennas are used in co-phase and spaced about 1/4 wavelength apart (just under 9ft - so not going to work well on a mini....), this means both are connected to the same centre feed wire in the co-ax cable, and share a common ground (usually vehicle body or the co-ax cable braid). For your needs one antenna is used as a ground plane (think of a battery - one positive terminal and one negative terminal - same with the antennas, one is "live" the other is ground, and completes a circuit).

Basically the two antennas are wired as an antenna called a dipole, which is basically two quater wavelength antennas back to back - here is an image:

http://images.google...h...ficial&sa=N

Both antennas are fed from the same co-ax cable that comes out of your CB, one is connected to the inner conductor (the "antenna") and the other is connected to the copper braid screening of the co-ax (the "ground plane"). Typically these would be joined together for mounting (but electrically seperated) to form one continuous long antenna that is fed at the mid point. This will not work on the mini, as the ground plane would be lower than the road...
So, the "ground plane" antenna will have to be mounted in the same way as the "transmitting" antenna, and in the same general area. You need to connect two co-ax feed lines that are identical in length, down to a fraction of an inch, one to either antenna (center wire of the co-ax is connected to the antenna whip on both), then both of these cables are connected to the single cable from your CB.

One of the antenna cables is connected to the centre conductor (centre conductor to centre conductor) AND the braid is connected to braid, the other is connected to the copper braid ONLY (centre to braid).

You will need a 'T' piece and apropriate connectors to connect these cables together - should be available from Maplins or any CB / Amature radio shops.



I'm trying to keep this simple to understand, but it might be easier to explain with a few pics - I'll see if I can dig something up.


*edit* just done a couple of measurments, a few calculations and a quick test........ the results are not spectacular. It's "useable" - as in it won't blow your rig up, but has horrible signal propagation, and questionable SWR and impedance values.

You *could* however run a co-phase setup (the mini is *just* long enough), one antenna at the front bumper, one at the back bumper - but that would look odd, and I'm not sure about the laws regarding a flexable antenna mounted at the front of a low vehicle such as the mini.....

Your only real choices are :

1) A whip antenna mounted on the roof, with four 9ft lengths of insulated wire running to oposite corners of the roof, and down A and C posts / looped around the interior roof area (this will require either drilling and mounting to the roof, or a mag mount with a large magnet on the underside of the roof to secure the antenna). OR mount a thin steel panel (about 9sqr ft / 3ft x3ft minimum) to the inside of the roof panel, then use a mag mount to avoid drilling the roof - this is prolly the better solution. Depending on your mounting position for the antenna, you should be able to run the insulated radial wires (ground plane) through the inside of the car, and hide them behind the interior trim. This is also the cheapest method.

2) A marine or "no ground plane" / electronic ground plane antenna. while this can be mounted almost anywhere on the car, because of the slightly lower results when using this method, I would still try to mount the antenna on the roof, or at least as high up as possible.

SS

Edited by Silicon Skum, 24 December 2007 - 12:28 PM.

[Big Man]

Hi There Stardue, Well I have two aerials, one is a five foot baseloaded whip (coiled at the bottom) but not a springer type! I use this on a mag mount on the roof when doing any distance, the second one is exactly the same but only three foot this on is mounted on a homemade braced discretly fixed to the left of the bootlid from under the bumper. Will get a pic downlo0aded if it helps but reception onb this one os reallt naff and need to sort out the earthing.

Cheers

BM

[666junky]

I didn't want to start a whole new topic over this.. so I thought id ask here..

Just bought myself a cb radio and aerial which is the bendy type with the coily bit at the bottom.. Now as I dont have my mini yet.. i was thinking how could i run it from home? i have a power supply suitable.. but im worried about all this "tuning" it in..and having to earth it.. and where do i put the aerial?

and exactly HOW important is it to tune the aerial? .. i was a bit cheeky and plugged it all in earlier.. even though it was on the ground in the back garden.. i still managed to pick up some random man talking.. is this ok just to run it like this?

As you can tell.. i know naff all about this thing!!

[Silicon Skum]

I didn't want to start a whole new topic over this.. so I thought id ask here..

Just bought myself a cb radio and aerial which is the bendy type with the coily bit at the bottom.. Now as I dont have my mini yet.. i was thinking how could i run it from home? i have a power supply suitable.. but im worried about all this "tuning" it in..and having to earth it.. and where do i put the aerial?

and exactly HOW important is it to tune the aerial? .. i was a bit cheeky and plugged it all in earlier.. even though it was on the ground in the back garden.. i still managed to pick up some random man talking.. is this ok just to run it like this?

As you can tell.. i know naff all about this thing!!

I just found this topic by accident - bit late, but here goes:

If you want to run the CB as a "base station" (ie in your house), you will need to give the antenna a ground plane (basically the biggest bit of metal sheet you can find - eg a car roof). I'm assuming that you have a mag mount, and in place of the large sheet of metal, you can use........A BISCUIT TIN! Seriously, all you need to do is mount the mag mount and antenna on the tin, drill four (4) holes, one at each corner of the tin, and then screw / solder four 9 foot long lengths of wire to the holes (any type will do, just as long as they are all 9ft long and have a GOOD ELECTRICAL CONNECTION to the tin - scrape the tin's paint off to clean metal!). Then you need to spread the wires out to the full length in oposite directions (so it forms a big cross). This is not a perfect antenna setup, but it's cheap, and works (and was used by a lot of people years ago). You will need to get the antenna up as high as possible, shove it into the attic under the roof, if you can. the higher it is, the better the signal you will get, and transmit.

Next you will need to tune the antenna - for this you will need an SWR meter. Instructions on using the meter are usually included, but it's just a simple matter of getting the needle to show as little as possible, when measuring the ammount of radio frequencey (RF) signal that BOUNCES BACK from the antenna INTO the CB. Ideal is 1.1:1 (meaning all is transmitted, very little reflected) but anything under 1.5 is generally safe enough not to do any damage to the CB (poor signal strength though).
If you have one of those "pre tuned" antennas, it will be REASONABLY SAFE to use the antenna (mounted on the biscuit / sweets tin) without tuning it (as long as it's in the same condition as you received it - meaning you haven't fiddled with it or adjusted it's length).

As for your current setup - yeah, it's fine for LISTENING, but don't try to transmit from the CB without a ground plane, it will damage or destroy the CB's final stage RF transistor (in other words, it'll 'brick' your CB).


Hope it's not too late, and good luck.

SS

[BigEnd]

anyone managed to get any pics yet?

I fitted this CB and aerial for shows and convoys etc. I choose the mag mount for easy removal. The CB is a Danita and is so small it fits easily under the drivers side dash and has 80 channels (40 UK and 40 European frequencies, ideal for this years BH2N ). Whole kit cost £95 new from CCC in Caerphilly.

[Bluejuice]

Got my CB from CCC in Caerphilly aswell. Really good service & know what they're talking about.

Fitted mine with a plug to take power from the cigarette lighter for easy fitting /removal.